There are an increasing number of radio applications requiring communication over a wide frequency spectrum, for example, 30 to 400 megahertz. Radios having circuitry which performs conversion of the received AM signal directly to the audio signal, without first mixing down to an intermediate frequency ("IF"), are commonly referred to as direct conversion, or zero-IF receivers.
In a typical IQ direct conversion receiver, incoming RF communication signals are split into a pair of equal amplitude components which are in phase with each other. These RF components are then mixed with separate injection signals at approximately the same frequency as the communications signal but which are 90.degree. out-of-phase with each other. I and Q baseband signal components which are in quadrature are thereby generated. These signals are then independently filtered and amplified at audio frequencies on separate signal channels. The I and Q components formed as a result of the mixing process allow the signal to be conveniently demodulated upon being supplied to a suitable signal processing unit.
This architecture works well except that it is very difficult to achieve and maintain identical gain and exact phase quadrature between the signal channels and variations between the signal channels which commonly occur as a result of changes in temperature, frequency and other operational parameters result in gain and phase mismatches which produce distortion products in the output receiver. Gain mismatches of as little as 0.2 dB and phase mismatches of as little as 1.degree. can result in distortion products which can not ordinarily be reduced to less than 30 to 40 dB in practice and correspond to discrete tones which greatly limit the performance of the receiver.
Various solutions to the problems are well known in the prior art based upon the generation of new quadrature and phase signals independent of amplitude effects. By then approximating DC components for I and Q, the original I and Q baseband signals are corrected for gain and phase errors between the signal channels. An example of such technical solution is set forth in U.S. Pat. No. 5,230,099, incorporated herein by reference.
Unfortunately, the prior art solutions to improving direct conversion receiver processing are ineffective in processing single side band suppressed carrier signals. While the prior art double sideband suppressed carrier signal solutions are relatively simple, a need exists for similar discoveries applicable to single sideband situations.